Generally, a direct-current (DC) offset occurs in a quadrature modulator or the like in a radio transmitting apparatus using direct radio frequency (RF) modulation that is used in a base station or the like. The DC offset makes a carrier leakage occur. The carrier leakage makes a quadrature modulation accuracy of a transmitting apparatus lower or makes a bit error rate of a receiving apparatus higher. The carrier leakage exerts a negative influence to adjacent bands in a wideband code division multiple access (W-CDMA) radio system.
Examples of a technique for correcting a DC offset include the following techniques (for example, see Japanese Laid-Open Patent Publication No. 09-83587 and International Publication Pamphlet No. WO2005/025168): (1) a technique in which a transmission signal is received with a feedback loop that is provided in a transmitter to obtain a feedback signal, in which a DC-offset component is extracted from only the received feedback signal, and in which a DC offset is corrected by a transmitting unit (a feedback (FB)-type DC-offset correction method or a feedback-signal-integral-type DC offset correction method); and (2) a technique in which a DC-offset component is extracted from the difference between the above-mentioned feedback signal and the transmission signal (a reference signal), and in which a DC offset is corrected by the transmitting unit (a reference-type DC-offset correction method or a signal-comparison-type DC offset correction method).
FIG. 7A is a diagram of a configuration of the related art.
A digital main transmission signal for an I channel that is used for an in-phase component and a digital main transmission signal for a Q channel that is used for a quadrature component, which are baseband signals to be transmitted, are input to DC-offset correction units 701 (#i) and 701 (#q), respectively. Output signals from the DC-offset correction units 701 (#i) and 701 (#q) are input to digital-to-analog converters (DACs) 702 (#i) and 702 (#q), respectively, to be converted into an analog main transmission signal for the I channel and an analog main transmission signal for the Q channel, which are baseband signals. The analog main transmission signal for I channel and the analog main transmission signal for Q channel are input to a quadrature modulator (MOD) 703.
The MOD 703 performs, on the basis of the analog main transmission signal for the I channel and the analog main transmission signal for the Q channel that are input from the DACs 702 (#i) and 702 (#q), respectively, quadrature modulation on a reference carrier wave that is output from an oscillator 704, thereby generating a transmission modulated signal.
Power amplification is performed on the transmission modulated signal by a power amplifier (PA) 705, and then, the transmission modulated signal is output to a transmitting-antenna feeding unit (not illustrated). Furthermore, the transmission modulated signal is input as a branch signal to a feedback system including units 706 to 711 by a directional coupler (not illustrated) or the like.
Frequency conversion is performed by a frequency conversion unit 706 on the branch signal using an oscillation signal that is output from an oscillator 707, thereby converting the frequency of the branch signal into an intermediate frequency or a baseband frequency.
The branch signal that was subjected to frequency conversion is converted into a digital signal by an analog-to-digital converter (ADC) 708. The digital signal that is obtained by conversion is converted into a feedback baseband signal for the I channel and a feedback baseband signal for the Q channel by a demodulator (DEM) 709 which operates on the basis of a signal that is output from a numerically controlled oscillator (NCO) 710.
The feedback baseband signal for the I channel and the feedback baseband signal for the Q channel are stored in an I-channel feedback-signal memory 711 (#i) and a Q-channel feedback-signal memory 711 (#q), respectively.
For example, in the above-mentioned technique (2), a central processing unit (CPU) 712 compares the feedback baseband signal for the I channel and the feedback baseband signal for the Q channel, which are stored in the I-channel feedback-signal memory 711 (#i) and the Q-channel feedback-signal memory 711 (#q), with a main transmission signal for the I channel and a main transmission signal for the Q channel, respectively, thereby detecting a carrier leakage that occurs in the DACs 702 (#i) and 702 (#q), the MOD 703, or the like. The CPU 712 calculates inverse components of the detected carrier leakage as a DC-offset correction value for the I channel and a DC-offset correction value for the Q channel. The CPU 712 inputs the DC-offset correction value for the I channel and the DC-offset correction value for the Q channel to the DC-offset correction units 701 (#i) and 701 (#q), respectively.
As illustrated in FIG. 7B, the DC-offset correction units 701 (#i) and 701 (#q) add the DC-offset correction value for the I channel and the DC-offset correction value for the Q channel to the main transmission signal for the I channel and the main transmission signal for the Q channel, respectively. The DC-offset correction units 701 (#i) and 701 (#q) output the main transmission signal for the I channel and the main transmission signal for the Q channel to the DAC 702 (#i) for the I channel and the DAC 702 (#q) for the Q channel, respectively.
As a result of the above-mentioned operation, carrier leakage comes not to be output in the output of PA 705.
In a configuration that is illustrated in FIGS. 7A and 7B, a DC-offset correction process that is performed by the DC-offset correction units 701 (#i) and 701 (#q) is performed in order to correct a carrier-leakage component that appears in a modulation frequency. However, there is a case in which an input signal having a amplitude of zero is input to a transmitting apparatus.
In this case, because the amplitude of the input signal is zero, it is difficult to calculate a phase difference from the difference between a feedback signal and a transmission signal. Thus, it is preferable that the DC-offset correction process be performed using the above-mentioned technique (1).
Furthermore, typically, it is difficult for the DAC 702 (#i) or 702 (#q), which are illustrated in FIG. 7A, to maintain complete linearity. As denoted by reference numeral 801 in FIG. 8, all output bits change at a point at which the value of an input signal changes from −1 to zero. Accordingly, a phenomenon in which an output changes by a large amount generally occurs at the point.
Characteristics associated with this phenomenon are stipulated as differential nonlinearity (DNL) characteristics, integral nonlinearity (INL) characteristics of a DAC, and so forth.
Additionally, regarding carrier-leakage characteristics in the MOD 703 illustrated in FIG. 7A, an optimum point (denoted by reference numeral 901 in FIG. 9) of the carrier-leakage characteristics changes for an input to the MOD 703 (an output from the DAC 702 (#i) or 702 (#q)) in accordance with the balance between the I channel side and the Q channel side as illustrated in FIG. 9.
Accordingly, using the relationships illustrated in FIGS. 8 and 9, the relationships between input signal that is input to the DAC 702 (#i) or 702 (#q) and carrier leakage are obtained as illustrated in FIG. 10.
The relative positional relationships between the input-output characteristics of the DAC 702 (#i) or 702 (#q) and the carrier-leakage characteristics in the MOD 703 or the like differ depending on a variation in production of elements constituting the DAC 702 (#i) or 702 (#q), the MOD 703, or the like, and are not easily predicted.
For example, when the positional relationships between the input-output characteristics of the DAC 702 (#i) or 702 (#q) and the carrier-leakage characteristics in the MOD 703 or the like are obtained as illustrated in FIG. 10, a value A can be calculated as an input to a DAC for an optimum point 1001 of the carrier-leakage characteristics. In this case, correction is performed by the DC-offset correction unit 701 (#i) or 701 (#q) illustrated in FIG. 7A so that an amplitude which is calculated as a DC-offset correction value is set to the value A, whereby the value of carrier leakage can be made to approach the optimum point 1001.
However, for example, as illustrated in FIG. 11, regarding the relative positional relationships between the input-output characteristics of the DAC 702 (#i) or 702 (#q) and the carrier-leakage characteristics in the MOD 703 or the like, in a case in which an optimum point 1101 of the carrier-leakage characteristics is positioned near a point at which the value of an input to a DAC is zero, even when DC-offset correction is performed and a DC-offset correction value is calculated as a result, the DC-offset correction value becomes zero. In other words, there is a case in which the value of carrier leakage that occurs when DC-offset correction is performed is equal to the value of carrier leakage that occurs when DC-offset correction is not performed. Accordingly, the value of carrier leakage cannot be made to approach the minimum point as illustrated in FIG. 11, and the related art has a problem that carrier leakage may not be reduced.
Consequently, the related art has a problem that the performance of a transmitting device largely depends on individual differences among DACs or MODs that occur in production.